Different questions, same conductor
How do you calculate wire size?
The size of an electrical conductor refers to its cross-sectional area (measured in circular mils (CM) or AWG in the US). The larger the cross-sectional area, the more current the wire can transmit with lower resistance—very similar to water flowing through a wider pipe with less pressure loss. We derive the required conductor area from Ohm's law and conductor resistivity:
where:
- V_drop – Voltage drop calculated between the supply source and the farthest end of the wire, measured in volts (V_drop = V_source × (% drop ÷ 100));
- I – Maximum current running through the conductor, measured in amps (convert power using the watts to amps calculator);
- K – Specific resistance constant of the conductor material at 75°C (12.9 Ω·CM/ft for copper and 21.2 Ω·CM/ft for aluminum);
- L – One-way length of the wire run from panel to load, measured in feet; and
- CM – Cross-sectional area of the conductor, measured in circular mils (where 1,000 CM = 1 kcmil).
Calculating wire sizes for three-phase AC
For a balanced three-phase AC circuit, three current-carrying phase conductors are used instead of a two-wire loop. The calculator accepts line-to-line voltage and line current, modifying the required area of a single conductor to:
The factor of √3 ≈ 1.732 converts between phase and line quantities in a balanced three-phase system, replacing the factor of 2. The calculator automatically selects the next AWG size up from its listed 14 AWG – 4/0 table so the voltage-drop limit is never exceeded. To check apparent power feeding that circuit, use the kVA to amps calculator.
| Check | Primary question | Key inputs | Typical failure |
|---|---|---|---|
| Ampacity | Can the conductor carry the load under its conditions of use? | Material, insulation, terminals, ambient, conductor count | Excess heating |
| Voltage drop | How much voltage is estimated to reach the load? | Voltage, current, distance, material, phase, area | Low delivered voltage or poor operation |
Worked example: a result near the boundary
For 240 V, 30 A, 100 ft one-way copper at a 3% target, 10 AWG produces about 3.11% in the site's simplified voltage-drop model. The calculator therefore moves to 8 AWG.
A table ampacity value viewed without terminal, load and installation context cannot reverse that voltage-drop observation. Likewise, selecting 8 AWG for drop does not prove its ampacity or protection is correct.
Recommended order of work
- 01
Determine the design load
Establish the current using the applicable equipment and load rules.
- 02
Establish minimum ampacity
Apply material, insulation, temperature, terminal, ambient and grouping conditions.
- 03
Check voltage drop
Use the actual voltage, distance, phase and material.
- 04
Choose the controlling size
Use a conductor that satisfies all applicable checks, then verify physical installation.
Warning signs
- A calculator result is being called an NEC-approved size
- Breaker rating is used as the only load input
- Long distance is ignored after an ampacity lookup
- The 90°C column is used without checking terminals
- Adjustment factors are mentioned but not calculated